The present invention relates to a charging member used in an image forming apparatus of a copier, a laser beam printer, a facsimile and the like, also to a charging device including the charging member, a process cartridge including the charging device and an image forming apparatus including the process cartridge.
A charging device that performs a charging process on an image carrier (photoreceptor drum) is used in a conventional electro-photo type image forming apparatus of an electronic photo-copier, a laser printer, a facsimile and the like. FIG. 8 is an explanatory diagram of an electrophotographic-type image forming apparatus having a conventional charger roller.
In FIG. 8, 130 is a conventional electrophotographic-type image forming apparatus. The conventional electrophotographic-type image forming apparatus 130 includes a photoreceptor drum 111 in which an electrostatic latent image is formed, a charging member (charger roller) 112 that performs the charging process by contacting the photoreceptor drum 111, an exposure device 113 of a laser beam or the like, a toner carrier (development roller) 114 that makes toner 115 to adhere onto the electrostatic latent image of the photoreceptor drum 111, a transfer member (transfer roller) 116 that transfers a toner image on the photoreceptor drum 111 onto a recording paper 117 and a cleaning member (cleaning blade) 118 for cleaning the photoreceptor drum 111 after the transfer process. In the FIG. 8, 119 are eliminated toners obtained from removing residual toners on a surface of the photoreceptor drum 111 by the cleaning member 118, 120 is an image development device and 121 is a cleaning device. In addition, in the FIG. 8, functional units normally necessary at other electrophotographic processes are not necessary in the present specification and are thereby abbreviated.
Next, basic image forming operations of the image forming apparatus 130 of the conventional electrophotographic-type will be described.
When DC voltage is fed from a power package (not illustrated) to the charger roller 112 which is in contact with the photoreceptor drum 111, the surface of the photoreceptor drum 111 is charged uniformly to a high electric potential. Soon afterwards, when image light is radiated by the exposure device 113 onto the surface of the photoreceptor drum 111, an electric potential of a part radiated by the photoreceptor drum 111 decreases. Such a charging mechanism by the charger roller 112 towards the surface of the photoreceptor drum 111 is known as an electrical discharge (spark discharge) based on Paschen's Law, in a minute space between the charger roller 112 and the photoreceptor drum 111.
Image light refers to light quantity distributions corresponding to black/white of an image. When the image light is radiated, due to radiation of the image light, an electrical potential distribution corresponding to a recorded image, that is, an electrostatic latent image is formed on a surface of the photoreceptor drum 111. Thus, when a part of the photoreceptor drum 111 having the electrostatic latent image formed passes through the development roller 114, toners are adhered corresponding to highs and lows of the electrical potential and a toner image visualizing the electrostatic latent image is formed. The recording paper 117 is carried at a predetermined timing by a registration roller (not illustrated) to the part of the photoreceptor drum 111 having the toner image formed to overlap the toner image. Then after the toner image is transferred onto the recording paper by the transfer roller 116, the recording paper 117 is separated from the photoreceptor drum 111. The separated recording paper 117 is carried through a paper path. The recording paper 117 is then fused by a fuser unit (not illustrated) to be ejected outside the machine. When transfer is finished in this manner, a cleaning process is performed on the surface of the photoreceptor drum 111 by the cleaning member 118. Furthermore, residual electric charges are removed by a quenching lamp (not illustrated) to prepare for the next image forming process.
As for a charging member used in the above-described image forming apparatus, a charging-by-contact type is widely used in which the charging member contacts the surface of the image carrier so that the surface of the image carrier is charged. There is a problem such that during usage of the charging member, adherent substances such as residual toners on the surface of the image carrier, toners deteriorated due to oxidized gas generated by the electrical discharge of the charging member and toner constituent substances or the like adhere to the surface of the charging member so that the charging member is smeared. Therefore, in order to remove these adherent substances, the cleaning member is disposed on the surface of the charging member. However, the cleaning member becomes smeared over time by the adherent substances removed from the charging member by the cleaning member so that cleaning capabilities of the cleaning member decrease. Therefore, electrical discharge irregularities are generated by the adherent substances adhered to the charging member and there is a problem such that abnormal images are generated.
Consequently, a charging-by-noncontact type charging member to charge the surface of the image carrier is gradually adopted in the above-described image forming apparatus in which the charging member is disposed to be in close contact with the image carrier with a certain space (minute gap) maintained between the charging member and the image carrier. In a case where the charging-by-noncontact type charging member is used, the charging member is not in direct contact with the image carrier so that it becomes difficult for the surface of the charging member to be tainted. Therefore, the charging-by-noncontact type charging member can have a longer life than the charging-by-contact type charging member.
As for the charging-by-noncontact type charging member, a minute gap is formed between the image carrier and the charging member and the minute gap is approximately equal to or less than 100 μm. Therefore, if a foreign substance such as a dust particle larger than the minute gap becomes trapped in the minute gap, there is a problem of generating an abnormal image caused by a poor electrification. In particular, in a part where the foreign substance is adhered, there is a problem of generating a black spotty image due to a low charged potential. In a charging member, a high AC (alternating-current) voltage is applied superimposedly to a DC (direct-current) voltage, therefore it is necessary for the charging member to have electrical conductivity. In addition, high voltage is applied to the charging member, and therefore a partial abnormal electrical discharge easily occurs due to a leak. In order to reduce the above-described abnormal electrical discharge, it is more preferable to use an ion-conductive charging member to which a conductive additive such as an electrolyte salt as a quaternary ammonium salt is added and a charging member made from an epichlorohydrin rubber having electrical conductivity than to use an electron-conductive charging member to which a conductive additive such as a carbon black is added.
FIG. 9 is a cross-sectional diagram of a conventional charging member. As illustrated in FIG. 9, a conventional charging member 112 has an electrically-conductive support 112c, an electrical resistance adjustment layer 112b provided on the electrically-conductive support 112c, and a surface layer 112a provided on the electrical resistance adjustment layer 112b (see Japanese patent application publication number 2008-111872). In the conventional charging member 112, the surface layer 112a is formed of an ion-conductive material, and the surface layer 112a is soft and has a high coefficient of friction. Therefore, when a foreign substance becomes trapped in a minute gap between the charging member 112 and an image carrier, the foreign substance is buried (adhered) in a surface of the charging member 112, because the surface of the image carrier is hard and has a low coefficient of friction. And therefore there is a problem such that even a cleaning member which is in contact with the charging member is not able to remove the foreign substance. Consequently, a technique such that a powdered solid lubricant is applied to the surface of the charging member and coefficient of friction of the surface of the charging member is decreased and thereby the foreign substance adhering to the surface of the charging member is prevented is disclosed in Japanese patent application publication number 2009-42550, and thereby some preventive effects are obtained, however it is not sufficient to prevent the foreign substance from adhering to the surface of the charging member. This is because a hardness of the surface layer of the charging member is lower than that of the surface layer of the image carrier, and therefore the foreign substance is pushed to a side of the charging member and the foreign substance is buried in the surface of the charging member and it is not possible for even the cleaning member to remove the foreign substance.